The present invention relates to an osteointegrative interface for implantable prostheses and to a method for the treatment of the osteointegrative interface, particularly to a biomimetic osteointegrative interface and to a method for modifying the surface of the osteointegrative interface superficially.
Biomimetic processes which simulate biomineralization processes have been used with success for preparing materials such as iron oxides, apatite, and cadmium sulphide which are used in various technological fields.
The field which has drawn most “inspiration” from biomineralization processes is the field of biomaterials and, in particular, biomaterials for orthopaedics, maxillo-facial surgery, and dentistry.
These materials have to interact with body tissues and should therefore possess particular biocompatibility and bioactivity qualities and mechanical properties.
The main uses of biomaterials in the hard tissues field include prostheses such as hip prostheses, knee prostheses, dental implants, screws, nails, plates and osteosynthesis means.
Because of the mechanical properties required, the materials usually used for prostheses are stainless steel, titanium, titanium alloys, and tantalum, which have the best biocompatibility characteristics of all of the metals.
These metals exhibit a high mechanical breaking load but require long implantation periods in order for their integration with the biological tissues such as, for example, bone, to be established.
To prevent this problem, there are known biomimetic treatments for osteointegrative interfaces, by means of which it is possible to bring about a significant increase in the rate of precipitation of hydroxyapatite after immersion in biological fluids, and its adhesion to the prosthesis.
Calcium phosphates, including hydroxyapatite, are the main constituents of the inert bone matrix.
U.S. Pat. No. 5,152,993 (Lars Magnus et al.) and U.S. Pat. No. 5,885,612 (Ohthuke et al.) propose a treatment in hydrogen peroxide with the use of hydrogen peroxide or hydrogen peroxide and metal ions, which modifies the surface of the prosthesis, promoting the formation of —OH hydroxyl functional groups which can induce the formation of a stable interface with bone.
U.S. Pat. No. 5,609,633 (Kokubo) and the corresponding application EP 678300A1 (Kokubo) teach a treatment based on immersion in an alkaline solution such as NaOH, KOH or CaOH2, followed by washing and high-temperature heat treatment.
In particular, these two documents describe a treatment for etching with hydroxides an oxide such as titanium oxide, which is almost completely crystalline and is composed mainly of the rutile, anastase and, rarely, brookite phases, which are different crystalline forms of titanium dioxide, that is TiO2.
This etching has the advantage not only of forming a larger number of —OH groups so that the surface layer hydroxylates more easily and is particularly significantly enriched with oxygen, but also of achieving the formation of a layer of amorphous calcium phosphates which are stoichiometrically similar to hydroxyapatite and can therefore promote the formation of mature hydroxyapatite.
The heat treatment included in this treatment with alkalis would lead to an increase of up to about 1 μm in the thickness of the titanium oxide layer.
To achieve a significant increase in the thickness of oxide which forms on the surface of a metal, anodic passivation treatments have been optimized by the development of a technique known as anodic spark deposition (ASD).
The ASD technique is a galvanic electrodeposition process which is performed at fairly high voltage and which causes point breakdown and perforation of the dielectric surface oxide layer which is formed progressively, thus allowing it to grow.
With this technique, as described, for example, in U.S. Pat. No. 5,385,662 (Kurze et al.), it is possible to produce coatings with the characteristics of sintered ceramic materials, which are particularly resistant to abrasion and corrosion, and with thicknesses of up to 150 μm.
With specific reference to bone implants, U.S. Pat. No. 5,478,237 (Ishizawa) teaches the use of this technique to bring about a modification of the composition, of the thickness, and of the morphology of the titanium oxide film, that is, the formation of a thick and nanoporous oxide film containing calcium and phosphorus, of a thickness clearly greater than that which is formed by natural oxidation of the metal.
However, this technique does not substantially improve the biomimetic properties of the titanium.
In order to improve its biomimetic capacity, the film or layer is subjected to a hydrothermic treatment which can promote nucleation of hydroxyapatite crystals. However, these crystals are not distributed homogeneously and do not cover the surface of the osteointegrative interface completely.
Homogeneity of behavior is therefore not achieved.
Moreover, the mechanical bond between oxide and crystals is very weak.